This invention relates to photoelectric incremental measuring apparatus for measuring the relative displacement of two objects, in particular to such incremental measuring apparatus as utilize position reference marks.
Various types of measuring apparatus are known in the art. For example, West German No. OS 18 14 785 discloses a photoelectric length-measuring apparatus which utilizes a position reference mark in addition to the measuring grid. It describes the operation of the apparatus and of the reference mark, and in FIG. 2 shows the structure of the reference mark.
It is also a known practice to provide a series of reference marks along the measuring grid of an incremental measuring system. From this series of like reference marks, individual marks can be selected for use by suitable means, as disclosed in West German No. AS 25 40 412.
Furthermore West German No. PS 876,162 discloses that particularly favorable results can be obtained if the lines of a group of lines that make up a reference mark are arranged at unequal distances from each other. In such a marking scheme the lines of the line groups on the reticle have the same spacing arrangement as the lines of the line groups on the scale. A signal with an improved signal-to-noise ratio is then obtained whenever the lines of the line groups on the reticle and scale come to overlap each other exactly.
The references discussed above identify a shortcoming of incremental measuring systems: after an interruption in operation, it is not directly possible to recover the initial position of the measuring system. Two of the references, Nos. OS 18 14 785 and AS 25 40 412, however describe how the initial position can be recovered.
The known measuring arrangements also have a disadvantage in that a partial failure of illumination in the system disrupts measurements and can lead to considerable interruptions in operation, with long down times.